Tire demounting machines are known to be equipped with a support arrangement for working tools, which are interchangeable according to the operations to be performed on vehicle wheel tires by tire repairers.
The support arrangement is mounted at the free end of an operating arm, which is hinged to a column that is part of the tire demounting machine, and rises upwards from a base designed to hold the motion imparting members of the tire mounting machine therein.
The operating arm may pivot in cantilever fashion in a plane of rotation parallel to the support surface of a wheel supporting unit which is mounted to the base and is known as a “self-centering unit”.
The support arrangement is integral with the end to which it is mounted and hence follows its rotary motion in a rigid manner, and changes its orientation relative to the “self-centering unit” when the operating arm is rotated through the various work positions.
The rotation angle of the arm typically changes with wheel diameters, and hence with tire diameters, whereby the working tools assume different work positions relative to the tires, according to the diameters of the latter, and only take substantially correct positions at certain diameters, typically intermediate diameters, whereas the work positions of the working tools at smaller or greater diameters are considerably deviated from the desired position, namely in directions converging towards the center relative to tire profiles, as tire diameters increase.
Particularly, for tire mounting and demounting, tire repairers use a tool that comprises a shank attached to the support arrangement and a first operating part designed to facilitate mounting of tires to rims, which has a substantially tapered profile with an end apex, and a second operating part designed to facilitate demounting of tires from rims, which has a positioning member projecting towards the self-centering unit and designed to contact one point of the rim edge and which is opposite to the first operating part on the tool.
Typically, to mount and demount tires to and from rims, tire repairers first lock the wheels on the self-centering unit and then maneuver the operating arm to lie the positioning element on the rim edge, while moving the working tool closer to the upper surface of the wheel locked on the self-centering unit whereupon they use a lever to cause first the upper bead of the tire, i.e. the one facing upwards, and then the lower bead, to pass over the demounting bead of the tool.
Each time that a bead passes over the tool, the self-centering unit is rotated about its axis of rotation and the tire is removed from its rim in two operating steps.
Likewise, to mount a tire to a wheel rim that has been previously locked on the self-centering unit, tire repairers first manually introduce a first lower bead into the central channel of the rim, and then maneuver the operating arm to place to tool close to the rim edge and to dispose the positioning element on the support arrangement.
Next, the repairers dispose a portion of the upper bead, which has not been introduced yet, on the mounting part of the working tool and use a lever to force a portion adjacent to the one lying on the mounting part underneath the edge of the rim, towards the central channel thereof, and rotate the self-centering unit.
Thus, the upper bead is progressively guided by the mounting part which elastically bends it as it runs thereunder, and is thereby forced to pass over the upper edge of the rim throughout the diameter of the bead towards the central channel.
Particularly, as viewed from above, the profile of the mounting part of a known tool for mounting and demounting tires to and from wheel rims, has an end with a tapered profile with an end tip which, at the start of each mounting step, is designed to wedge between the lifted bead of the tire and the rim edge to assist the bead to follow an inclined guide surface formed in the tool as an extension of the tip, according to the direction of rotation of the self-centering unit.
This surface has such a shape as to guide the bead which, due to the rotation of the self-centering unit, slides thereon, and to force it to pass over the rim edge, thereby introducing it into the central channel of the rim, as mentioned above.
This prior art has a drawback typically consisting in that the support arrangements are mounted to their respective ends of the support arms in a fixed manner, as mentioned above, which causes the orientation of the parts for mounting and demounting tires on rims to change according to wheel diameters, thereby progressively increasing the difficulty of the operations as the orientation of the working tool deviates from the best operating position, due to too large or too small displacements of the operating arm.
More in detail, the tip of the mounting part progressively moves away from the inner profile line of a tire bead when the latter, as mentioned above, has a very small or a very large diameter.
As distance between the tip and the bead profile line increases, the steps of lifting the bead and introducing the tip between the bead and the rim edge become increasingly difficult, because the bead profile has to be deformed to a greater extent in the portion of the tire sidewall where mounting beings, to lay the bead on the part of the tool that is designed therefor.
Such step is both laborious and poorly stable because, due to the elasticity and high stiffness of the beads of certain types of tires, the excessive deformation created by the tire repairer on the bead using the lever tends to cause the bead to slip off the working tool, thereby making it useless, and forcing the tire repairer to repeat the lifting step possibly several times to mount a tire to a rim, using the tire demounting machine.